The present invention relates in general to an electric motor driver for driving an electric motor, and more particularly to an electric motor driver which includes in the inside thereof a current control loop, with which an electric motor current is detected to form the feedback control unit, and which is suitable for controlling a position, velocity, a current, a torque and the like.
As the microprocessors have advanced, in the electric motor control apparatuses as well for use in the general industrial machines, the conventional analog control has been changed into the digital control employing the micro-controller. The devices and components which are used in the printed circuit boards have also been changed from the transistors to the IC (Integrated Circuit) and further to the MSI (Medium Scale Integrated Circuit), the LSI (Large Scale Integrated Circuit) and the VLSI (Very Large Scale Integrated Circuit) so that the ASIC (Application Specific IC), the dedicated customs LSI which is ordered by the customer, the gate arrays and the like have been used. Thus, the miniatulization of the controllers have advanced. Also, the ministulization of the package of the digital IC has further advanced by adopting the technique of mounting them to the surface of the printed circuit board. In addition, as the high density integration of the ICs has advanced, the number of lead terminals of the IC chip (the number of pins) has exceeded 100, and along with this, instead of the conventional IC including the lead terminals which have the pitch of 2.54 mm, the IC chip including the lead terminals which have the pitch of 1.27 mm called the half pitch, and further the IC chip including the lead terminals which have 1.00 mm, 0.8 mm or 0.75 mm pitch have been used. In addition, in recent years, the lead package having the pitch of 0.5 mm has also been used. In correspondence thereto, for the conductor pattern spacing of the printed circuit board, in recent years, the design has been advanced from the construction in which one conductor pattern is passed through the space defined between the adjacent lead terminals having the pitch of 2.54 mm to the construction in which three conductor patterns or three or more conductor patterns are passed through the space defined between the adjacent lead terminals having the pitch of 2.54 mm. Thus, the high density mounting for the printed circuit board has advanced.
On the other hand, with respect to the power source for subjecting the general industrial machine to the variable speed running, the vector control without any of velocity sensors with which the induction motor as the A.C. motor is driven by the inverter, and the vector control with a velocity sensor are adopted, and also the A.C. servo motor, which self-contains a position sensor and a velocity sensor, for use in the machine tools, the metal processing machines, the assembly machines, the spinning machines, the weaving machines, the robots, or the like has been actively used, from the request for the automation and the labor saving, in the processing/assembling field or the like. The basic control thereof is the position, velocity, current and torque control. From the request for the promotion of the high speed for the line velocity and the promotion of the high speed for a tact time in recent years, in the vector control for the induction motor, the high speed current control is carried out in such a way that the current of the motor is separated into the current component for the torque which is in proportion to the torque and another current component for the magnetic flux which intersects perpendicularly the current component for the torque. In addition, in the case of the A.C. servo motor, for the revolving-field type synchronous motor employing the permanent magnet as the rotor, the position of the permanent magnet is detected by the magnetic pole positions detector (rotor position detector) in order to carry out the high speed current control. In any case, the instantaneous value of the current is controlled.
The place where each of those A.C. motor controllers is installed is located within the control board which is installed in the vicinity of the motor in the assemble field or the processing field, and hence in the environment in the periphery of the control board, the scrap metal which has been generated by the cutting and the cutting oil are floated in the air, and in the case of the tire processing factory, the dust containing therein carbon particles which has been generated in processing rubber is floated in the air, and also in the case of the textile factory or the spinning machine factory, the cotton dust is floated in the air. Those carbon dust and cotton dust are adhered little by little to the printed circuit board provided within the controller. In addition, in the weaving machine factory, in the water jet style wherein the weft yarns of the automatic weaving machine for knitting the warps and the weft yeans are blown by the water, since the water is treated, the humidity in the periphery thereof is increased, and the dew condensation is generated on the printed circuit board of the motor control circuit as the temperature falls gradually. The ICs are mounted on the printed circuit board and the above-mentioned devices having the small lead pitch are mounted thereon. For this reason, the leads of the ICs will be covered with the dust containing carbon or the cotton dust which have been adhered to the printed circuit board with lapse of time.
When the running of the A.C. motor is started in the morning everyday, upon the start of the running, the temperature in the control board rises, and then when the running of the A.C. motor is stopped in the night, the temperature in the control board falls and the external air temperature also falls. For this reason, the humidity of the air, within the control board, which is heated in the daytime is increased and when it becomes the night, the dew condensation occurs on the circuit board with the dust containing carbon and the cotton dust at the temperature equal to or lower than the critical temperature. Then, if at the time when starting the running in the morning on the next day, the supply of the electric power to the circuit board is started, then the electrical short circuit may occur between the adjacent leads of the ICs in some cases. While this electrical short circuit is apt to occur in the rainy season. In the case of the weaving machine of the water jet style in which the water is used, the electrical short circuit occurs irrespective of any of the seasons. For this reason, the operation of the motor control circuit may lead to the malfunction, and the semiconductor devices in the inverter main circuit may be broken down in some cases.
Next, the description will hereinafter be described in detail with respect to the influences in the case where the adjacent leads are electrically short circuited resulting from that the dust and the cotton dust have been adhered to the lead terminals of the ICs mounted on the printed circuit board of the A.C. servo motor control circuit for controlling the position, velocity, current and torque, and also the moisture is bedewed together with the dust and the cotton dust.
FIG. 1 is a control block diagram, partly in circuit diagram, showing a configuration of an A.C. servo motor as a conventional one example. In the figure, reference numeral 1 designates a servo motor, and reference numeral 2 designates an encoder which includes complexly a magnetic pole positions sensor 2-1 for detecting the magnetic pole positions of a rotor of the A.C servo motor and a rotational speed and position sensor 2-2 for detecting the rotational speed and position of the A.C. servo motor, and also which is self-contained in the A.C. servo motor. Reference numeral 3 designates a position control unit, reference numeral 5 designates a velocity control unit, reference numeral 7 designates a current control unit, and reference numeral 8 designates a PWM (Pulse Width Modulation) control computing unit. These constituent elements 3, 5, 7 and 8 constitute the forward element. Also, reference numeral 4 designates a position counter circuit, reference numeral 6 designates a velocity computing circuit, and reference numeral 10 designates a current detection circuit. In this connection, these constituent elements constitute the feedback elements of the position, velocity and current feedback loops, respectively. Reference numeral 9 designates a magnetic pole positions computing circuit for receiving, as its inputs, the signals from the magnetic pole positions sensor (rotor position sensor) 2-1 and the rotational speed and position sensor 2-2 for the rotor of the A.C. servo motor 1 to compute the positions of the magnetic poles of the rotor. Reference numeral 13 designates an inverter main circuit which includes a driver for driving each of switching devices and a control power source circuit therefor. Of those constituent elements, the inverter main circuit is mounted in the power module which is molded with resin such as epoxy resin. Reference numeral 11 designates a current detector for detecting the current which is caused to flow through the A.C. servo motor to feed the current thus detected back to a current detection circuit 10. In this connection, reference numerals 12-1 to 12-3 designate respectively summing points at each of which the sign + represents the addition and the sign - represents the subtraction. In FIG. 1, since the command signal is denoted by the sign + and the feedback is denoted by the sign -, the constituent elements associated with the feedback constitute the negative feedback control circuit.
First of all, a position command .theta.ref is issued to this controller 25, and then this position command .theta.ref is compared with a position feedback value .theta.f, which has been detected in the position counter circuit 4 on the basis of the signal outputted from the rotational speed and position sensor 2-2 of the encoder 2, in a comparator 12-1. The position deviation as this result of the comparison is inputted to the position control unit 3 and then is outputted in the form of a velocity command Nref from the unit 3. This output signal is further compared with a velocity feedback value Nf, which has been computed in the velocity computing circuit 6 on the basis of the signal outputted from the rotational speed and position sensor 2-2, in a comparator 12-2 and then the resultant deviation is inputted to the velocity control unit 5. The output signal is outputted in the form of a current command Iref from the velocity control unit 5 and then is compared with a current feedback value If, which has been obtained through the current detector 11 and the current detection circuit 10, in a comparator 12-3. Then, the resultant deviation is inputted to the current control unit 7. The output signal of the current control unit 7 becomes a PWM signal through the PWM control unit 8 to be supplied to the power switching device 13 so that the current is supplied to the A.C. servo motor 1 to control the A.C. servo motor 1.
Incidentally, since the permanent magnet type A.C. servo motor requires the positions of the magnetic poles of the motor, the positions of the magnetic poles are obtained in the magnetic pole positions computing circuit 9 on the basis of the signal outputted from the magnetic pole positions sensor 2-1 of the encoder 2, and the signal exhibiting the the positions of the magnetic poles is inputted to the current control unit 7 to be used to carry out the control.
Now, the three kinds of feedback loops, i.e., the position feedback loop, the velocity feedback loop and the current (torque) feedback loop are formed. In general, with respect to the responsibility of each of the feedback loops, the responsibility is made rapid and also the stable control system which has the small overshoot and undershoot is formed as the loop is formed in the more inner side. The responsibility is, in the case of the velocity control, set 4 to 6 times as large as that of the position control, and the responsibility of the current control is set 4 to 6 times as that of the velocity control. For this reason, when the software processing is executed in the microprocessor CPU, in general, the computing period of the software is most rapid in the current feedback loop, and the computing period of the software is next in speed that of the current feedback loop, and also the processing speed of the position feedback loop is lowest.
FIG. 2 is a block diagram showing a configuration of an example of the general external protection form, i.e., an external protection apparatus 18 which is provided in the outside of the A.C. servo motor 1, the encoder 2 and the A.C. servo motor controller 20. The apparatus 18 is the apparatus for striving for the safety of the workers when the malfunction occurs in the A.C. servo motor. In the apparatus 18, the protection system corresponding to the machine is installed. Such sensors of the protection system include an acceleration sensor 14 which is installed on the machine side, an over velocity sensor 15, and right-hand and left-hand edge sensors 16 which are respectively mounted to the both sides of the operation range. Then, when having exceeded the specified acceleration or when having exceeded the specified velocity, or when having exceeded the specified operation range of the machine, the output signals which have been outputted from those sensors are respectively inputted to a protection processing circuit 17 for preventing the run away to stop the operation of the A.C. servo motor. In addition, by operating this external protection apparatus 18, the safety management, such as the operation of turning OFF the main circuit power source and the operation of putting ON the machine brake, is carried out. In general, the A.C. servo motor 1, the encoder 2 and the A.C. servo motor controller 20 are all manufactured as the general purpose products by the electrical machinery and apparatus manufacturers, while the set makers purchase those electrical machinery and apparatuses to carry out the mechanical design in order to install the external protection system for the overall system including the electricities and the machines.
FIG. 3 is a circuit diagram showing a configuration of an internal circuit of an example of a power module which is used in the conventional A.C. servo motor controller. In the figure, A.C. power sources are connected to terminals R, S and T, respectively, and each of power source voltages is converted from an A.C. form to a D.C. form by a diode rectifier 43. A current limiting resistor (not shown) which is provided in the outside of the power module is connected between terminals P and P1, while a ballast capacitor (not shown) is connected between the terminals P1 and N so that the rush current which is caused to flow through the ballast capacitor when turning ON the A.C. power source is limited by the current limiting resistor. The D.C. voltage which has been smoothed by the ballast capacitor is then applied to an inverter circuit 45 which is constituted by six sets of inverse parallel circuits consisting of switching devices Su, Sv, Sw, Sx, Sy and Sz (power transistors and the like) and diodes Du, Dv, Dw, Dx, Dy and Dz. By enabling/disabling the base or the gate of each of the switching devices, A.C. voltages is outputted to drive an A.C. servo motor (not shown) which is connected to output terminals U, V and W. In addition, the energy which is regenerated from the A.C. servo motor is accumulated in the ballast capacitor which is connected between the terminals P1 and N, and when the D.C. voltage has been increased, a switching device of a regenerative braking circuit 44 is turned ON together with the discharge resistor (not shown) which is connected between the terminals P1 and BR, and the regenerative energy is consumed through the discharge resistor. The gate input signals of the seven switching devices of the regenerative braking circuit and the inverter circuit are supplied from a driver circuit which is provided in the outside of the power module. These gate terminals GU, EU, GV, EV, GW, EW, GX, GY, GZ and E are, as shown in a power module 22 of FIG. 4 which will be described later, the fine lead terminals and are arranged at small pitches. On the other hand, the main circuit terminals are thick lead terminals and are arranged at large spacing through which the dust may be readily passed. In the main circuit printed circuit board, those lead terminals are passed therethrough in correspondence to the terminal positions of the power module 22 and are electrically connected thereto by the soldering. The lead pitch of the gate terminals of the main circuit board becomes similarly small, and if absolutely similarly to the lead pitch of the IC, the electric power is supplied in the state in which the moisture is bedewed together with the dust and the cotton dust, the electrical short circuit occurs between the adjacent leads of the gate terminals of the power module in some cases.
FIG. 4 is an exploded view showing the construction of an example of a conventional A.C. servo motor controller. In the figure, reference numeral 22 designates the power module which has been described with reference to FIG. 3 and which is mounted to a cooling fin 21. Reference numeral 19 designates a logic printed circuit board, and reference numeral 23 designates a main circuit on a printed circuit board. Reference numeral 24 designates a cover with which the power module 22 and the two sheets of printed circuit boards 19 and 23 are covered. Ventilating holes 39 are bored through the cover so that the air which has been heated in the apparatus can be exhausted to the outside in a naturally air-cooling manner. In addition, through holes 40, 41 and 42 are formed in the front face in order to introduce therethrough a main circuit connector 26 which is mounted to the main circuit printed circuit board 23, an I/O connector 37 which is mounted to the logic printed circuit board 19, and an encoder connector 38, respectively.
The logic printed circuit board 19 corresponds to the board on which the constituent devices are mounted which are surrounded by the frame denoted by reference numeral 19 in FIG. 1, and which includes the IC devises such as one microprocessor (CPU) 31 for controlling and computing the positions, the velocity, the current (torque) and the like, an A/D converter 32 for converting the analog current of the A.C. servo motor into the digital current, a gate array 33, a ROM (Read Only Memory) 34, an interface IC 35 and a RAM (Random Access Memory) 36.
On the main circuit printed circuit board 23, there are mounted the main circuit components such as the current limiting resistor connected between the output terminal P of the diode rectifier 43 for converting the A.C. form to the D.C. form and the terminal P1, the ballast capacitor 25 connected between the terminals P1 and N and the two current detectors 11 for detecting the current of the A.C. servo motor 1, and also a switching transformer 27, an IC 29 for the switching power source, a drive circuit for driving the signal, which has been outputted from the PWM control computing unit, using a driver IC 28, an interface IC 30 for exchanging the interface with the main circuit, and the like which constitute the control power source circuit. In such a way, the various kinds of IC devices and components are mounted on the logic printed circuit board 19 and the main circuit printed circuit board 23 shown in Fog. 4, and for the lead pitches of those IC, LSI, VLSI, gate array, and ASIC, the high density package having the lead pitch of about 0.5 to 0.8 mm is adopted.
Next, the description will hereinbelow be given with respect to the operation in the case where the dust and the cotton dust have been adhered to the lead terminals of the IC which is mounted on the printed circuit board and the moisture is bedewed together with the dust and the cotton dust so that the adjacent lead terminals are electrically short circuited. Considering the case where the adjacent lead terminals are electrically short circuited, while the adjacent lead terminal is at the logic power source voltage of 5V (at the fixed electric potential of the high level H), or at the common 0V (at the fixed electric potential of the low level L), or it is another signal line (the electric potential H and the electric potential L are mixedly present), simply the description will now be given with respect to the case where the logic power source voltage of 5V (fixed to the electric potential H) and the common 0V (fixed to the electric potential L) are present.
The discussion will now be given with respect to the operation in the case where the electrical short circuit occurs on the line at 0V which is, in general, common to the signals of the automatic control loop having the feedback. When in FIG. 1, the position feedback signal .theta.f, the velocity feedback signal Nf and the current feedback signal If which are all the feedback signals on the minus side of the summing points 12-1 to 12-3 electrically short circuit with the 0V line at the common electric potential, the feedback control is carried out in such a way that in response to the command signals .theta.ref, Nref and Iref, the actual position, velocity and current (torque) follow the command values, respectively.
First of all, when the position feedback signal .theta.f of the position control loop electrically short circuits with the .theta.V line, it is judged that the A.C. servo motor 1 has not yet reached the command position though in actual, the A.C servo motor 1 has already reached that position, and as a result, the acceleration is made. However, since the position feedback signal electrically short circuits with the 0V line, the information relating to the proper position is not fed back and as a result, the acceleration is continuously made to cause the run away. However, if the current control loop is properly operated, then the current of the A.C. servo motor 1 is limited to level equal to or lower than the maximum value and hence the external protection apparatus 18 is operated. Therefore, the A.C. servo motor 1, the encoder 2 and the A.C. servo motor controller 20 can be all prevented from being broken down at all.
Next, when the velocity feedback signal Nf of the velocity control loop electrically short circuits with the 0V line, it is judged that the velocity of the A.C. servo motor 1 has not yet reached the command velocity though in actual, the velocity of the A.C. servo motor 1 has already reached that velocity to increase the velocity of the A.C. servo motor 1. However, since the velocity feedback signal electrically short circuits with the 0V line, the information relating to the proper velocity is not fed back and as a result, the velocity is continuously increased to cause the run away. In this case as well, if the current control loop is properly operated, then the current of the A.C. servo motor 1 is limited to the level equal to or lower than the maximum value. Therefore, the A.C. servo motor 1, the encoder 2 and the A.C. servo motor controller 20 can be all prevented from being broken down at all.
Next, when the current feedback signal If of the current control loop electrically short circuits with the 0V line, it is judged that the current of the A.C. servo motor 1 has not yet reached the command current though in actual, the current of the A.C. servo motor 1 has already reached that current to increase the current in an instant. However, since the current feedback signal electrically short circuits with the 0V line, the information relating to the proper current is not fed back and as a result, the current of the A.C. servo motor 1 is continuously increased to exceed the maximum current in an instant to become the overcurrent, and hence there is no time for accelerating the A.C. servo motor 1 to carry out the operation of protecting the over-current detection circuit so that each of the switching devices of the inverter main circuit interrupts its base and the A.C. servo motor controller 20 trips. At this time, the current which is caused to flow through each of the switching devices of the power module 22 is increased up to the large saturation current which is determined on the basis of the current amplification factor of each of the switching devices and as a result. the over-current detection circuit is operated to interrupt that current. At this time, the temperature in the inside of each of the switching devices has risen, and hence that situation is repeatedly caused in the state in which the internal temperature have not yet fallen, then the each of the switching devices will be broken down due to the thermal fatigue.
The response time in the position control loop and the velocity control loop is the acceleration/deceleration time for moving actually the machine. Then, the acceleration/deceleration time is in proportion to the sum of the moment of inertia of the A.C. servo motor 1 and the moment of inertia of the machine which is obtained in the term of the motor axis and also is in inverse proportion to the difference between the motor torque and the load torque. Therefore, the moment of inertia of the machine is related thereto and hence the responsibility is slow. In addition, as described above, in order to configure the stable control system which is less in the overshoot and the undershoot, the order of decreasing the responsibility is the current loop, the velocity loop and the position loop. Therefore, the design is made in such a way that the response time of the position loop and the velocity loop is made slower than that of the current loop. For this reason, an acceleration sensor, an over velocity sensor and right and left-hand sides edge sensors which are mounted to the both ends of the operating range so as for the A.C. servo motor not to depart the operating range are provided in the machine, whereby the respective factors can be detected without any of the time delay and hence the machine can be protected. For example, the set makers manufacture the external protection apparatus 18 for the overall system including the electricity and the machinery, and the run away is prevented from being caused by the provision of the acceleration sensor 14 and the over velocity sensor 15 which are installed on the machine side, and the right and left-hand sides edge sensors 16 which are mounted to the both ends of the operating range, whereby the machine can be protected. Therefore, it is possible to protect the A.C. servo motor 1.
In addition, the discussion will hereinbelow be given with respect to the operation in the case where the electrical short circuit occurs in the 5V line on which the level of each of the signals in the loops in the feedback system becomes the level H.
When the position feedback signal 0f, the velocity feedback signal Nf or the current feedback signal If which is added on the minus side of one of the summing points 12-1 to 12-3 electrically short circuits with the 5V line at the level H, i.e., the maximum value in the positive direction, the feedback control is carried out in such a way that in response to the command signals ref, Nref and Iref, the actual position, velocity and current (torque) follow the command values, respectively.
First of all, in the position control loop, since though the A.C. servo motor 1 has actually already reached the specified position, the information relating to the positive maximum position is fed back, it is judged that the A.C. servo motor 1 has already been rotated beyond the specified position, and hence the acceleration is made in the reversing direction. However, since the position feedback signal electrically short circuits with 5V line at the level H, the information relating to the proper position is not fed back and hence the acceleration is continuously made in the reversing direction to cause the run away.
Next, in the velocity control loop, it is judged that though in actual, the velocity of the A.C. servo motor 1 has already reached the command velocity, the velocity of the A.C. servo motor 1 has exceeded that velocity to reach the maximum velocity, and as a result, the velocity of the A.C. servo motor 1 is decelerated to be accelerated in the reversing direction. However, since the velocity feedback signal electrically short circuits with the 5V line, the information relating to the proper velocity is not fed back and hence the acceleration is continuously made in the reversing direction to cause the run away state. However, if the current control loop is properly operated similarly to the case where the feedback signal electrically short circuits with 0V line, then the current of the A.C. servo motor 1 is limited to the level equal to or lower than the maximum value, and hence the A.C. servo motor 1, the encoder 2 and the A.C. servo motor controller 20 can be all prevented from being broken down. In addition, the external protection apparatus 18 for the overall system is manufactured, and the acceleration sensor 14, and the over velocity sensor 15 which are installed on the machine side, and the right and lefthand sides edge sensors 16 which are mounted to the both ends of the operating range are provided, whereby the run away is prevented from being caused to ensure the safety and to protect the machine. Therefore, it is possible to protect the A.C. servo motor 1.
Next, in the current control loop, it is judged that though in actual, the current of the A.C. servo motor 1 has already reached the command current, the current of the A.C. servo motor has reached the maximum current to decrease the current in an instant, and even when has exceeded the negative maximum current on the reverse polarity side, the current of the A.C. servo motor 1 is still increased. However, since the current feedback signal electrically short circuits with the 5V line, the information relating to the proper current is not fed back and hence the current of the A.C. servo motor 1 is continuously increased to the negative side to be the over current in an instant. If that situation is rapidly caused, then each of the switching devices will be broken down due to the thermal fatigue in an instant.
In this connection, the detection of the disconnection of the output lines to the magnetic poles positions sensor 2-1 and the rotational speed and position sensor 2-2 of the encoder 2, and the A.C. servo motor controller 20 is described in JP-U-62-44262. Then, the pulse encoder employing both of the reference pulse and the inverted pulse which is obtained by inverting the reference pulse is used, and the detection of the disconnection is carried out on the basis of the exclusive OR of both of the pulses on the reception side. As a result, upon the disconnection of the encoder output lines from the encoder 2 to the A.C. servo motor controller 20, the disconnection is detected in an instant, and hence it is impossible to cause the run away of the machine at all.
Next, the description will hereinbelow be given with respect to the case where the electrical short circuit occurs with the line at the level L or H on the forward element side from the summing points 12-1 to 1203 shown in FIG. 1.
When the output of the position control computing unit 3 or the velocity control computing unit 5 electrically short circuits with the line at the level L, the velocity command Nref or the current command Iref becomes zero in the level. When the output of the position control computing unit 3 electrically short circuits therewith, the velocity of the A.C. servo motor is decelerated so that the A.C. servo motor is stopped. In addition, when the output of the velocity control computing unit 5 electrically short circuits therewith, the A.C. servo motor becomes the free run state and hence is not broken down at all.
Next, when the electrical short circuit occurs with the line at the level H, the level of the each of the velocity command Nref and the current command Iref becomes the level H. When the output of the position control computing unit 3 electrically short circuits with the line at the level H, the A.C. servo motor is run at the maximum velocity, and while when the output of the velocity control computing unit 5 electrically short circuits with the line at the level H, the maximum current is caused to flow through the A.C. servo motor to accelerate the velocity of the A.C. servo motor, the A.C. servo motor can be protected by the operation of the external protection apparatus 18.
Next, let us consider the case where the output of the current control unit 7 electrically short circuits with the line at the level L or H, if the output of the current control unit 7 electrically short circuits with the line at the level L, then the A.C. servo motor becomes the free run state, while if the output of the current control unit 7 electrically short circuits with the line at the level H, then the level of the voltage command becomes maximum, and the current exceeds the maximum current to be the over-current so that the A.C. servo motor trips. At this time, the temperature in the inside of each of the switching devices has risen, and hence if that situation is repeatedly caused in the state in which the internal temperature has not yet fallen, then each of the switching devices will be broken down due to the thermal fatigue.
In addition, when in the output of the PWM control computing unit 8, the electrical short circuit occurs between the adjacent pins, normally, for the PWM signal, the upper arm switching device and the lower arm switching device are alternately, repeatedly turned ON and OFF. Then, if the electrical short circuit occurs, then this short circuit becomes the arm short circuit which electrically short circuits across the ballast capacitor of the inverter circuit. In this case as well, if such a situation is repeatedly caused, then the each of the switching devices will be broken down due to the thermal fatigue.
From the foregoing, even if the dust and the cotton dust have been adhered to the lead terminals of the position control loop and the velocity control loop of the IC which is mounted on the printed circuit board and then the moisture is bedewed together with the dust and the cotton dust so that the electrical short circuit occurs between the adjacent lead terminals, then the current of the A.C. servo motor is controlled equal to or lower than the maximum current and hence the power module can be prevented from being broken down at all. If the printed circuit board is dried, then the A.C. servo motor may be run again in the normal state in some cases. However, at the time when the dust and the cotton dust have been adhered to the lead terminals of the current control loop of the IC and then the moisture is bedewed together with the dust and the cotton dust, the current of the A.C. servo motor 1 exceeds the maximum current in an instant. Therefore, if such a situation is repeatedly caused, then the power module will be broken down due to the thermal fatigue. For this reason, only the current control loop is most sensitive to the breakdown of the power module, and hence in the case where the associated printed circuit board is installed in the place having the bad circumstance and also the dew condensation is generated, there is the much dangerousness that the current control loop may be broken down.
In this connection, in accordance with the official gazette of JP-A-6-169578, since the printed circuit board is mounted on the power module and also the printed circuit board is not fixed thereto by epoxy resin, there is the possibility that the dust and the cotton dust are adhered between the lead terminals of the IC and the like and then the moisture is bedewed together with the dust and the cotton dust. For this reason, in the portion in which the lead pitch is small, the protection is not sufficient for occurrence of the electrical short circuit. In addition, in accordance with the official gazette of JP-A-9-229972, the microprocessor executes the processing of controlling the position, the processing of controlling the velocity, and the processing of controlling the current, and the current feedback data is transmitted through the data bus which is distributed among the data buses of other loops. Therefore, in the case where the dust and the cotton dust have been adhered to the microprocessor CPU, the A/D converter or the printed circuit board and then the moisture is bedewed together with the dust and the cotton dust, the devices are arranged in which the pitch of the lead terminals or the spacing between the conductor patterns on the printed circuit board is small, and hence the electrical short circuit may occur. This problem is not taken into consideration in the prior art at all.
JP-A-9-65662 teaches that a current sensor is provided for detecting the current flowing through a power semiconductor element, the current sensor is disposed together with a main circuit in a module, and the detected signal is outputted to an external circuit separate from the module. This document does not concern a short circuit in the current control system.
JP-A-4-304184 teaches separate two CPUs one of which controls the current control system and the other controls the speed control system. The data is interchanged between the CPUS. This document does not concern a short circuit in the current control system.